In a related mobile communication system, when a plurality of base stations is arranged, the reception level of a mobile station may sometimes be decreased around the boundary of a cell that is formed by each of the base stations. Furthermore, in an area that includes a plurality of base stations, if mobile stations are distributed in an unbalanced manner, user load is concentrated on a specific base station; therefore, the throughput of the mobile stations connected to the base station may sometimes be decreased. Such a decrease in the reception level or the throughput causes a decrease in the communication quality of, for example, the mobile station located near the edge of the cell.
As a countermeasure against the problem described above, there is a method that improves the communication quality described above by arranging a centralized control station that controls the plurality of base stations in the mobile communication system and by optimizing, performed by the centralized control station in accordance with the distribution of the mobile stations, the transmission power of each of the base stations and association of each of the mobile stations with the base stations to which the mobile stations are connected. With this method, first, the centralized control station collects measurement information, for example, reference signal received power (RSRP), from the mobile stations connected to the base station that manages the mobile station and calculates, based on the measurement information, a path loss between a base station and a mobile station. Then, the centralized control station obtains a transmission power value indicating that the index value, such as the communication quality at the edge of a cell or the like, is the maximum; obtains the mobile station serving as the counterpart of the connection; and then notifies the obtained results to each of the base stations. Then, in accordance with the notification, each of the base stations changes its own transmission power and changes, as needed, association of a mobile station to other base station, which corresponds to a handover (HO) process.
There are two types of handover as follows. A first handover is a handover starting from a mobile station (hereinafter, referred to as a “normal handover”). With the normal handover, if a predetermined trigger condition is satisfied by a decrease in the reception level from its serving base station or the like, a mobile station reports the measurement information described above to the base station. The base station decides a target base station for handover based on the reported measurement information and then hands over the mobile station to the target base station.
In contrast, a second handover is a handover starting from a base station (hereinafter, referred to as a “forced handover”). With the forced handover, the base station forcedly hands over a mobile station to the base station that is indicated by the centralized control station regardless of whether the predetermined trigger condition is satisfied.
Patent Document 1: Japanese Laid-open Patent Publication No. 2012-100220
Patent Document 2: Japanese Laid-open Patent Publication No. 2011-35763
Non-Patent Document 1: Kobayashi Takaharu, Kimura Dai, Seki Hiroyuki, “A study of Downlink Transmission Power Control in Femtocell Networks”, the Institute of Electronics, Information and Communication Engineers (IEICE) Society Conference, Fujitsu laboratories Ltd., p. 430, Sep. 13 to 16, 2011
However, there are the following problems with the above described handovers.
First, with the normal handover, a mobile station does not perform a handover process unless the predetermined trigger condition is satisfied. Consequently, there may be a case in which the accuracy of the optimization is decreased and thus the communication quality may sometimes be decreased. In the following, a description will be given in detail below with reference to FIG. 8. FIG. 8 is a schematic diagram illustrating a problem of a normal handover performed in a related technology. In FIG. 8, it is assumed of a case in which, in order to optimize the state, the centralized control station reduces the transmission power of a handover source base station 100 and increases the transmission power of a handover destination base station 200. Consequently, a cell C100 formed by the base station 100 becomes smaller and a cell C200 formed by the base station 200 becomes larger; therefore, from among the four mobile stations connected to the base station 100, a mobile station U200 located at the edge of the cell is connected to, i.e., handed over to, the base station 200.
However, if optimization is performed under the assumption that a mobile station connects to the base station from which received power is the maximum, when the difference between the reception levels received by like the mobile station U100 from the base stations 100 and 200 is very small (RSRP200<RSRP100), a predetermined trigger condition, such as RSRP200>RSRP100+an offset value, is not satisfied. Consequently, even though the mobile station U100 is located at the edge of each of the cells C100 and C200, the mobile station U100 is not connected to the base station 200 that is the handover destination indicated by the centralized control station. Namely, the mobile station U100 remains in the base station 100 even though the mobile station U100 is preferably handed over to the base station 200 under normal circumstances in terms of load distribution. However, if optimization is performed by taking into consideration the trigger condition, a handover is performed only when the difference of the reception levels between the base stations 100 and 200 is equal to or greater than an offset value, which decreases the resolution of the control. This is a cause of a decrease in the accuracy of the optimization of the normal handover and, furthermore, a cause of a decrease in the communication quality.
In contrast, with the forced handover, a base station forcedly performs the handover process regardless of a change in the communication status due to movement of a mobile station. Consequently, even if a handover to a base station indicated by the centralized control station is not appropriate any more, the handover is performed and thus the communication quality may sometimes be decreased. In the following, a description will be given in detail below with reference to FIG. 9. FIG. 9 is a schematic diagram illustrating a problem of a forced handover performed in the related technology. In FIG. 9, it is assumed of a case in which the centralized control station collects measurement information from a mobile station U300 that is connected to a base station 300 and the mobile station U300 moves by the time at which the centralized control station indicates the base station functioning as the handover destination to the base stations 300, 400, and 500.
With the forced handover, the mobile station U300 is handed over to, for example, a base station 400 that forms a cell C400 regardless of the communication status at the time of the execution of the handover. However, if the mobile station U300 moves in the direction indicated by an arrow Y1 or Y2, the reception level of the cells C300 and C500 becomes higher than the reception level of the cell C400. Consequently, the mobile station U300 is handed over to the base station 400 that forms the cell C400 in which the reception level that is lower than that previously used and, on the contrary, the communication quality is decreased.
Furthermore, with the normal handover, it is possible to perform a handover that takes into consideration the communication status at the time of the execution of the handover; however, as described above, a mobile station does not perform the handover process unless the above described predetermined trigger condition is satisfied.
As described above, with the normal handover and the forced handover, there may be a case in which a mobile station is not handed over to an appropriate base station or is handed over to an inappropriate base station. Consequently, the quality of the communication between the base station and the mobile station may possibly be decreased.